It is well known in the art to make hollow plastic products such as containers or other like products using the extrusion blow-molding process. In the extrusion blow-molding process, plastic in a melted state is introduced into a mold. The molten plastic is generally extruded into a cylindrically-shaped tube or hose referred to as a parison. The mold is closed and the interior of the mold is then pressurized to force the melted plastic onto the interior walls of the mold, which is then allowed to cool so that the plastic solidifies. The end product is a hollow plastic product having the shape of the interior of the mold. In some applications, such as the production of fuel tanks for automobiles, the parisons may be 8 feet or more in length.
In the extrusion blow-molding process, the parison is formed by pressing the melted plastic through a die in one of two ways, either by the accumulator method or by the continuous extrusion method. In the accumulator method, a hydraulic means intermittently forces the melted plastic through the die to form the parisons. In the continuous method, the melted plastic flows continuously through the die to form the parison.
Previously, only small and mid-size plastic hollow products could be produced using the continuous extrusion blow-molding method. Recent developments in the art of blow-molding have allowed the production of larger sized products using the continuous method. In order to produce the larger containers, however, correspondingly larger equipment is required. As well, the parison must be correspondingly larger in diameter and length.
For this reason, two different versions of the continuous extrusion blow-molding process are known. In the first version, the die is located above the mold unit and the parison is transported vertically down into the mold unit. The disadvantage with this version of the method is that the equipment is very tall where large products are to be produced. In some applications, the equipment can be as high as 40 to 45 feet. Therefore, much taller buildings are required to house the equipment.
A further disadvantage of the vertical transportation version of the continuous process is that the parison must be carried into the mold unit, which is now done by pinching the upper end of the parison, separating it from the die and lowering it into the mold unit. This pinching and vertical movement, however, often causes the larger sized parisons to collapse or flatten onto each other. That is, the pinching of the upper end causes the parison wall to collapse throughout the entire length of the parison rather than the parison retaining its generally cylindrical shape. This creates further difficulties in the overall manufacturing process since the lower end of the parison closes, thereby preventing the parison from being located properly in the mold area. By "pinching" it is meant that the wall of the parison is closed.
The second version of the continuous extrusion process includes shuttling of the machine clamping units. In this method, at least two mold units mounted on individual clamping units are used for each parison-forming die. The die is typically located between the two or more mold units, and the clamping units are each moved in turn under the die so the mold units can receive the parison. Thus, this method requires a substantial amount of floor space in order to operate the equipment and accommodate the two or more clamping units. As well, the clamping units used for larger products can be very heavy (up to 100,000 lbs each), therefore they must be mounted on moving steel frame structures with drives and rails. This, of course, makes the equipment relatively very expensive. As well, the power consumption required to accelerate, move and stop the units is great.
Various devices for carrying parisons are disclosed in the art. In U.S. Pat. No. 3,000,051, a method and apparatus for manufacturing hollow articles from melted plastic tubes (i.e. parisons) is disclosed. Also disclosed is a method and apparatus for handling a heated, vertically extruded tube or rod of thermal plastic material prior to enclosing the tube into a mold. The parison is extruded between two vertically-spaced pairs of horizontally aligned holding members. The two lower holding members are provided in opposed vertical surfaces with identical tube-engaging notches having a generally tapered configuration. The parison is extruded and the bottom tube holding members are moved toward each other until they abut. At that point, the cooperating notches define an aperture that is somewhat smaller than the diameter of the extruded parison so that the lower portions of the parison are grasped with sufficient firmness by the holding members to provide vertical support for the parison. On the other hand, the top pair of tube holding members also have tube engaging notches, but the aperture defined by the top tube holding members is slightly larger than the diameter of the parison so that the top holding members do not engage the parison wall. Therefore, only the bottom pair of tube holding members actually engage and vertically support the parison. The top tube holding members do not engage the parison wall. As well, the parison is only moved vertically from the extruder to the mold; there is no mention of the holding members being capable of moving the parison horizontally.
U.S. Pat. No. 4,153,408 discloses a gripping device for holding the parison, and a method for extrusion blowing hollow articles in which the parison is transferred from the extrusion die to a blow mold by means of the gripper device. The gripper device includes gripper jaws that grip the parison as it issues from the extruder. The gripper jaws move in scissor-like fashion so that the top of the parison is pinched by the gripper jaws. Therefore, this gripper device requires the parison to be pinched in order to be retained within the gripping device.
U.S. Pat. No. 4,761,130 discloses a method and apparatus for blow-molding square-shaped articles. The parison must be pinched at both ends. U.S Pat. No. 4,770,839 also discloses a blow-molding method in which the parison must be pinched.
The present invention eliminates the disadvantages of the prior art, and of the vertical transportation and shuttling versions of the continuous extrusion blow-molding process known so far. The present invention provides a parison carrying device that carries the parison to the mold unit without moving the mold units themselves, without requiring the mold unit to be located immediately under the die and without pinching the parison. The parison carrying device of the present invention includes a collar means that retains the parison without pinching the parison wall thus minimizing the likelihood of the parison collapsing. The present invention also provides means for closing the upper end of the parison without necessarily pinching the parison wall in applications where the interior of the parison is pressurized prior to the mold closing.